1. Field of the Invention
The present invention relates to linear variable differential transducer (LVDT) assemblies.
2. General Background and State of the Art
A linear variable differential transformer (LVDT) is a displacement transducer that produces an electrical signal proportional to the displacement of a moveable core (armature) within a cylindrical transformer. The transformer consists of a central, primary coil winding and two secondary coil windings on opposite ends of the primary winding. The coil windings are coaxial. The armature preferably is nickel-iron and is positioned within the coil assembly. The core provides a path for magnetic flux linking the primary coil to the secondary coils.
When the primary coil is energized with an alternating current, a cylindrical flux field is produced over the length of the armature. This flux field produces a voltage in each of the two secondary coils that varies as a function of the armature position. Armature movement moves the flux field into one secondary and out of the other causing an increase in the voltage induced in one secondary and a corresponding voltage decrease in the other. The secondary coils are normally connected in series with opposing phase. The net output of the LVDT is the difference between the two secondary voltages. When the armature is positioned symmetrically relative to the two secondary windings (the xe2x80x9cnullxe2x80x9d position), the differential output is approximately zero, because the voltage of each secondary is equal but of opposite phase.
Subjecting an actuator to pressure or force can move an LVDT armature through a linkage. Thus, LVDTs are commonly used in actuators. As pressure increases, the armature moves toward one secondary winding and away from the other. This yields a voltage difference that can be proportional to the linear movement. Consequently, this voltage output can measure pressure and position.
Nearly all LVDTs that are designed for aircraft or missile applications are wound on an insulated stainless steel spool, magnetically shielded and enclosed in a stainless steel housing using welded construction. The armature is normally made from a 50% nickel-iron alloy and brazed to a stainless steel extension. Secondary leads are usually shielded to minimize channel-to-channel cross talk for multi-channel units and to shield components from RF energy.
The length and diameter of an LVDT must be sufficient to allow adequate winding space for achieving the desired electrical performance, support any pressure requirement and withstand the environmental shock, vibration and acceleration. Where physical size is limited, electrical performance must be flexible. Although the LVDT is basically a simple device, the operating characteristics and electrical parameters are complex and depend to a large extent on the physical limitations.
U.S. patent application Ser. No. 09/547,511, filed Apr. 12, 2000, discusses some of the parameters that designers consider when specifying the sizes of LVDT components. That discussion and the remained of the application are incorporated by reference.
An LVDT""s output voltage is proportional to the voltage applied to the primary. System accuracy depends on providing a constant input to the primary or compensating for variations of the input by using ratio techniques. The output can be taken as the differential voltage or, with a center tap, as two separate secondary voltages whose difference is a function of the displacement. If the sum of the secondary voltages is designed to be a specific ratio of the difference voltage, overall accuracy significantly improves.
LVDT assemblies are commonly used to provide an electrical output indicating the position of a moving part, such as a mechanical linkage on an airplane. Operating two or more LVDT units in parallel with their electrical outputs combined and averaged yields a more accurate indication of the position of the linkage.
In the initial installation or coupling of the LVDT assembly to the linkage, the input to the LVDT assembly may be an externally threaded rotary head. During assembly, the rotary head is threaded into a threaded hole in the linkage while the rest of the LVDT assembly remains stationary.
It is important that the rotary coupling between the input member and the remainder of the LVDT assembly have low backlash. Reduced backlash insures accurate indication of the position of the moving linkage despite back and forth movement of the linkage. Heretofore, the rotary coupling has used two conventional ball bearing assemblies oppositely stressed. The prior art also has used spring washers or thrust bearings to reduce the backlash.
The LVDT assembly of the present invention employs a single ball bearing assembly and a single metal ball to accomplish the same low backlash function with the same reliability as the more complex constructions of the prior art.
In accordance with a specific illustrative embodiment of the invention, an LVDT assembly includes multiple LVDT units mounted in parallel to an LVDT input member for concurrent operation. The LVDT assembly measures movement of a linkage. The LVDT assembly also includes a rotary head that has a threaded portion for securing to a linkage. The rotary head is secured to the linkage by rotating the head and engaging mating threads on the linkage and on the rotary head. A conventional ball bearing assembly is provided between the rotary head and the stationary input member. A metallic ball is between the rotary head and the input member. A set screw engages the ball. Tightening the set screw stresses the ball bearing. The stress substantially eliminates backlash between the rotary head and the LVDT input member.
In accordance with a further aspect of the invention, the rotary head may have another type of linkage engaging portion such as a rotary quick latch; and force may be applied to the ball by a tapered wedging action mechanism, instead of the set screw.
Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description, and the accompanying drawings.